Right-clicking on most blocks with a water bucket places water on the side of the hitbox you're hovering over. For waterloggable blocks like fences, it would place the water there. However, with signs, it doesn't waterlog as expected, but it doesn't place the water anywhere or empty the bucket.
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It's not, this one's about water and that bug is related to dye.
The reason why you are unable to empty the water bucket on the sign is because the hand animation is played when attempting to do so, therefore preventing you from performing this action.

Sorry, I'm not sure I understand. Isn't the hand animation always played when placing blocks?

What I believe he / she is saying is that you are right clicking the sign, as signs have the capability to be right clicked. (Probably for command block reasons.) Therefore you are right clicking the sign, rather than placing down a water bucket.
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It could be partially for command block reasons, but probably's mostly for the capability to be right-clicked also for coloring the text of the sign.
Thanks for the explanation! While this and MC-166885 are related, I still wouldn't consider them duplicates.
I'm going to resolve this as a duplicate of MC-166885, as that is what caused this issue. When that issue is fixed this will likely be resolved as well, so there's no need to keep multiple tickets open about the same underlying issue ๐
Duplicate ofย MC-166885.